CODEWITHSUNDEEP
c
Era
Era

Reputation: 23

How to select an array with a variable?

I would like to select an array with a variable in order to copy its values into another array.

I'm struggling with this line : xyz[i] = arr1[i];

I want to replace arr1 with the variable name

Something like I use in Bash : xyz[i] = ${name}[i];

Does anyone have any clues or solutions ?

Here's a simplified version of my code

#include <stdio.h>

int main() {
    int arr1[8] = {1, 2, 3, 4, 5, 6, 7, 8};
    int arr2[8] = {8, 7, 6, 5, 4, 3, 2, 1};

    int xyz [8], i, num=8;
    int loop;
    char name[4];


    printf("Array's name ? ");
    scanf("%s", &name);

    // Array's name ? arr1

    for (i = 0; i < num; i++) {
        xyz[i] = arr1[i];
    }

    for(loop = 0; loop < 8; loop++) {
        printf("%d ", xyz[loop]);
    }

    // 1 2 3 4 5 6 7 8

   return 0;
}

Upvotes: 0

Views: 883

Answers (2)

John Bode
John Bode

Reputation: 123508

C doesn't provide this capability (after the source code has been compiled, variable names no longer exist as such). You'll need to use a pointer to do something like this, and you'll need to add logic to assign the right value to that pointer. For a couple of names, a simple if/else statement is good enough:

int *p = NULL;   // p will point to the first element of arr1 or arr2 depending on the value of name

if ( strcmp( name, "arr1" ) == 0 )
  p = arr1;
else if ( strcmp( name, "arr2" ) == 0 )
  p = arr2;
else
  fprintf( stderr, "%s is not a valid name\n", name );

if ( !p )
{
  // handle bad entry
}
else
{
  for ( i = 0; i < num; i++ )
    xyz[i] = p[i];
}

In the general case (where you have more than just a few options), you'll want to build some kind of a map that associates a name with an address:

struct {
  char *name;
  int *arr;
} name_arr_map[] = { 
  {"arr1", arr1}, 
  {"arr2", arr2}, 
  ... 
};

int *p = NULL;

// get name

for ( i = 0; i < N; i++ ) // N entries in our map
{
  if ( strcmp( name_arr_map[i].name, name ) == 0 )
  {
    p = name_arr_map[i].arr;
    break;
  }
}

if ( !p )
{
  // handle bad entry
}
else
{
  for ( i = 0; i < num; i++ )
    xyz[i] = p[i];
}

For a few entries (a couple of dozen or so) a simple linear search like this is good enough. If you have more than that, you may want to use a more sophisticated structure like a hash table or a tree.

However, for this specific case...

When you find yourself creating multiple variables of the same type with the names thing1, thing2, etc., that's a real strong hint you want an array - in this case, a 2D array:

int arrs[2][8] = { { 1, 2, 3, 4, 5, 6, 7, 8 },
                   { 8, 7, 6, 5, 4, 3, 2, 1 } };

In this case you don't need a name, just an index:

int idx = 0;
...
printf( "Which array do you want to use, 0 or 1? " );
scanf( "%d", &idx );
if ( idx < 0 || idx > 1 )
{
  // handle bad entry
}
else
{
  for ( i = 0; i < 8; i++ )
    xyz[i] = arrs[idx][i];
}

Upvotes: 2

Aplet123
Aplet123

Reputation: 35522

C does not have the ability to do this. You can either manually do it with if/else statements or find some sort of hashmap implementation in c like this one in order to map from names to arrays. Note that C++ has a pre-installed hashmap implementation that you don't have to download.

Upvotes: 1

Related Questions